The conjugated diene polymers such as a styrene/butadiene rubber and a nitrile rubber have a wide range of applications. These conjugated diene polymers have been generally produced by a radical polymerization method using a polymerization initiator such as an azo initiator, an organic peroxide, a persulphate or the like.
However, in radical polymerization of a conjugated diene, it has been difficult to control a molecular weight, a molecular weight distribution, a branch structure, and the like of a conjugated diene polymer to be obtained. Therefore, there have been involved such a problem that the conjugated diene polymer to be obtained is inferior in moldability, controllability during molding, and the like.
In order to widen the application range of a conjugated diene polymer to various materials, a conjugated diene polymer having a desired functional group at a terminal of a polymer chain is desired. However, it has been difficult to obtain directly such a polymer by a radical polymerization method.
A method of obtaining a conjugated diene polymer, subsequently introducing a halogen atom to a terminal of the polymer chain, and then converting the halogen atom into various functional groups can be considered for the introduction of a desired functional group to a terminal of a polymer chain.
For the production of the polymer bearing a halogen atom at a terminal of a polymer chain, there has been known a method of converting the hydroxy group of the copolymer having a hydroxy group at a terminal of a polymer chain into a halogen atom (see, for example, PATENT LITERATURE 1).
This method, however, has a defect that, since the halogen atom adds to the olefin structure moiety of the main chain as well as a terminal of a polymer chain upon replacing the hydroxy group at a terminal of a polymer chain by a halogen atom, inherent characteristics of the conjugated diene polymer are lost.
Therefore, although a halogen atom can be introduced into a terminal of a polymer chain, the resulting polymer is inferior in moldability and controllability during molding, and, hence, there is involved such a problem that the application range as materials is limited.
On the other hand, according to an anion polymerization method applied to the production of styrene/butadiene rubber and the like, a conjugated diene polymer, having a molecular weight, a molecular weight distribution, a branch structure and the like controlled, can be obtained. The conjugated diene polymer thus obtained is excellent in moldability and controllability during molding. In the anion polymerization method, however, the kind of the functional group capable of being introduced to a terminal of a polymer chain is limited, and, hence, it has been difficult to introduce a high reactive halogen atom into a terminal of a polymer chain.
For that reason, after the introduction of for example, a hydroxy group, a step such as replacement of the hydroxy group by a halogen atom is required, and a hydrogen halide or a thionyl halide must be employed for this replacement. On this replacement, a part of halogen atoms partly add to an olefin structure moiety of a main chain as well as polymer chain terminals, which causes a problem that inherent characteristics of a conjugated diene polymer are lost.
Even in this method, therefore, although a halogen atom can be introduced to a terminal of a polymer chain, the polymer obtained is inferior in moldability and controllability during molding, and, hence, there is involved such a problem that the application range as materials is limited.
In recent years, a polymerization method called living radical polymerization has been studied actively. In living radical polymerization, the activity of a polymerization terminal is not damaged, and molecular weight and molecular weight distribution can be controlled. A polymer bearing a halogen atom at a terminal of a polymer chain can be easily obtained, for example, by polymerizing a radically-polymerizable monomer by living radical polymerization using a polymerization initiator comprising a transition metal complex and an organic halide compound or a sulfonyl halide compound (see, for example, PATENT LITERATURE 2). However, in living radical polymerization, depending on the kind of the monomers to be used, it has been difficult to produce a polymer having a narrow molecular weight distribution while controlling its molecular weight.
Other living radical polymerization methods such as reversible addition-fragmentation chain-transfer polymerization using a thiocarbonyl compound as a chain-transfer agent, nitroxide-mediated polymerization using a radical scavenger such as a nitroxide compound, and the like have been known. In these polymerization methods, use of a halogen-substituted thiocarbonyl compound and a halogen-substituted nitroxide compound permits production of a polymer bearing a halogen atom at a terminal of a polymer chain (see, for example, PATENT LITERATURES 3 and 4). However, since these polymerization methods cause a decrease in the polymerizability of a conjugated diene, it has been very difficult to produce a conjugated diene polymer having a narrow molecular weight distribution while controlling a molecular weight.
As stated above, by these polymerization methods known so far, it has been extremely difficult to obtain a conjugated diene polymer which have a halogen atom at a terminal of a polymer chain and have a desired molecular weight and a desired narrow molecular weight distribution.